The intermetallic compound triniobium tin, Nb.sub.3 Sn, is a type-II metallic superconductor of interest because it has high values of superconducting critical current density in high magnetic fields. In order to achieve high critical current density, the process chosen to form the triniobium tin superconductor is important. One process currently used is a liquid-solid phase diffusion method. This occurs by diffusion between a solid niobium phase and a liquid tin phase.
To form triniobium tin superconductors by liquid-solid diffusion requires multiple steps. The first step in forming triniobium tin superconductor is to clean the niobium based substrate. Historically, this is done with a cleaning solution or etchant, such as a mixture of nitric acid, hydrochloric acid, and water. Diluted hydrofluoric acid is also sometimes used for cleaning the substrate. Another method of cleaning the foil is mechanical abrasion of the surface of the foil, the subject of commonly owned and assigned, Patent Application entitled "Cleaning Method for Foil" application Ser. No. 08/415,804. After the substrate is cleaned, oxygen may be added to the surface of the substrate by anodizing the surface electrolytically.
The next three steps involve high temperature heat treatments. The first anneal, as taught by Caslaw in British patent 1,342,726, is used to introduce a desired oxygen content into the niobium substrate. This is accomplished by passing the substrate through a furnace at about 950.degree. C. for about 30 seconds in an atmosphere containing argon and oxygen. However, if the substrate has been previously anodized to form an oxide layer on the surface of the substrate, then the preheat is called a decomposition anneal whereby the substrate is annealed so that the oxide layer diffuses into the body of the substrate.
After the preheat, the substrate is dipped in a tin or tin alloy bath, which supplies the tin for the triniobium tin reaction. The tin coating from the bath has a limiting thickness due to the amount of tin needed to further react with the niobium. Subsequently, the tin coated niobium substrate is treated with a reaction anneal to react the tin coating and the niobium base metal. During this final anneal, a layer of superconducting triniobium tin is formed on both sides of the niobium substrate.
In the above-mentioned steps the initial cleaning of the foil surface is important. A clean, unstained foil surface allows successful subsequent processing of the foil to form the triniobium tin superconductor. When the foil is not properly cleaned, surface staining from incomplete rinsing and drying of the foil after acid cleaning may occur. Stained foil cannot successfully be processed through all the necessary steps to make satisfactory triniobium tin tape. For instance, the amount of oxygen added during a subsequent anodization process is diminished in stained areas, reducing the thickness of the superconducting layer and lowering the critical current in those stained areas. Further, the tin alloy, which is also necessary for the formation of the superconductor, does not always wet the niobium foil surface in stained areas. This leaves areas on the foil where there is no superconducting material formed during the final reaction anneal.
There is a need for a method of cleaning the foil which would provide foil with a contaminant-free, uniform surface for subsequent superconductor process steps. There is also a need for a cleaning method that is compatible with the environment by eliminating the use of acids. Also, there is a need to reduce the cost of the manufactured tape by increasing the superconducting material yield.